A sample of a completed invitation with all of its enclosures should be brought to the post office for a clerk to actually weigh. A reminder that the weight of an invitation is not the only variable that determines the postage due on your invitations. The overall dimensions of the outer envelope, although within standard weight requirements, may require additional postage. So, it is always suggested to let the post office see the invitation ensemble before affixing postage. This will also give you the opportunity to choose custom stamps appropriate for your taste.